The present invention relates to a multilayer wiring substrate suitable for mounting a plurality of electronic elements thereon by a thermocompression bonding technique.
A prior-art-multilayer-wiring substrate using an organic material as an insulating material, which is suitable for finer and denser wiring and for decreasing capacitance between signal lines, has been proposed by Dr. M. TERASAWA et al. in a paper entitled "A Comparison of Thin Film, Thick Film, and Co-Fired High Density Ceramic Multilayer with The Combined Technology: T&T HDCM", published in October 1983 in the International Journal for Hybrid Microelectronics, Vol. 6, No. 1, pp. 607 to 615. However, the hardness of such organic materials (about 70-150 vickers) is extremely low as compared with that of inorganic materials such as alumina (Al.sub.2 O.sub.3) (about 1,500 Vickers). As a result, when wires connected to a large scale integrated circuit (LSI) chip are connected to pads on a substrate by thermocompression bonding under high temperature and pressure, the organic material is caved in or otherwise deformed not only to prevent normal bonding but also to affect the underlying wiring layers to cause poor wirings.
In order to eliminate such disadvantages in the prior art, U.S. Pat. No. 4,578,304 proposes a wiring substrate comprising a special metal layer 3 disposed beneath bonding pads 1. When extremely high pressure is accidentally applied onto the pads 1 at the time of bonding, however, contact between the pads 1 and the metal layer 3 or contact among the pads 1, the metal layer 3 and the uppermost wiring layer 7 may be caused to form unintended wiring routes.